1. Field of Invention
The present invention relates to a light source device of a type such that emission is outputted by way of a scattering guide and, in particular, to the device of said type capable of providing an improved uniformity in color temperature of emission. The light source device in accordance with the present invention is applied advantageously to backlighting of display such as liquid crystal display, display for advertising or decoration. Besides, the present invention is applicable advantageously not only to backlighting but also to various illumination devices of other types.
2. Related Art
A known type of light source device employs a scattering guide provided with an end face portion to which light is supplied and a side portion from which illumination light is emitted. Employable scattering guides have shapes such as wedge-plate, flat plate with uniform thickness and rod-shape.
FIG. 1 is a broken perspective view illustrating a light source device which employs a wedge-plate-like scattering guide (called "scattering guide plate", hereafter). Such a type of light source device is called "light source device of side light type" since a primary light source for light supply is disposed beside the scattering guide.
Referring to the figure, light source device 1 comprises a scattering guide 2 having a wedge-shaped cross section beside which a primary light source 3 is disposed.
The primary light source 3 is composed of, for instance, a cold cathode lamp 9 and a reflector 10. The light source device further comprises a reflection sheet 4, prism sheets 5, 6, which are arranged laminatedly together with the scattering guide 2. When the light source device 1 is applied to backlighting of a liquid crystal display, a liquid crystal display panel 7 is disposed on the outside.
The primary light source 3 includes a cold cathode lamp (fluorescent lamp) 9 backed by a reflector 10. The reflector 10 has an opening through which illumination light is supplied to a side end face 2a of the guide plate 2. The reflector 10 is, for instance, made of a sheet having regular or irregular reflectivity. The scattering guide plate 2 having a wedge-shaped cross section is, for instance, an injection moldings made of an acrylic resin (PMMA resin). Illumination light is introduced into the scattering guide plate 2 propagates with repeating reflections at a major face, along which the reflection sheet 4 is disposed (called "back face", hereafter), and at another major face, along which the prism sheet 5 is disposed (called "emission face", hereafter).
At every chance of the reflections, components having incidence angles smaller than the critical angle with respect to the emission face is emitted from the emission face and the back face. Illumination light outputted from the emission face has a principal propagation direction inclined toward the distal end of the wedge-shape. This property is called "emission directivity ".
The reflection sheet 4 is a sheet-like regular reflection member such as metal foil or a sheet-like irregular reflection member such as white PET film. The reflection sheet 4 reflects illumination light leaked from the back face of the scattering guide 2 to return the light into the scattering guide 2, thereby improving utilization efficiency of illumination light.
The prism sheets 5 and 6 are disposed to correct emission directivity of the scattering guide 2 and are light-permeable sheets made of, for instance, polycarbonate. Prismatic surfaces of the prism sheets 5, 6 are formed on faces (inside face) directed to the scattering guide plate 2 or on faces (outside face) opposite with the scattering guide plate 2. Each prismatic surface consists of a great number of projection rows running generally in parallel with each other.
If two prism sheets 5, 6 are employed as illustrated, one prism sheet 5 (or 6) is orientated usually so that its projection rows are in line with an incidence face of the scattering guide plate 2. And the other prism sheet 6 (or 5) is orientated usually so that its projection rows run perpendicularly to the incidence face (end face receiving light supply) of the scattering guide plate 2.
Each projection row of the prism sheet 5 and 6 is provided with slopes which corrects a principal emitting direction of emission toward the frontal direction of the emission face. A single prism sheet may be employed or, in some cases, no prism sheet may be employed. Otherwise may be employed a so-called double-prism-faced sheet having prismatic surfaces on both faces.
Furthermore, the scattering guide 2 may be like a uniform-thick plate alternatively. However, viewing from a standpoint of light utilization efficiency, wedge-shape is preferably adopted as in the illustrated example. In general, the wedge-shaped scattering guide 2 has a tendency such that thickness decreases according to distance from the end face portion 2a which receives light supply.
One of performances requisite to such a type of light source device is uniformity in tint of emission. That is, it is desired that the emission face of the scattering guide plate 2 provides emission which is even in tint over an area wide as possible. However, a conventional light source devices as above-described is not enough to answer this requirement. And some unevenness remains in the output light from the scattering guide plate or in the final output emitted by way of an additional element such as a prism sheet.
Careful observation on the emission face 2 employed in the conventional light source device 1 brought a finding that unevenness in tint has a certain tendency. That is, unevenness in tint tends to appear as a variation of tint depending on distance from the end face portion 2a.
For the sake of explanation, the emission face is divided into three areas, neighbouring area 2b near to the end face portion 2a, remote area 2d far from the end face portion 2a and intermediate area 2c located between them. According to observation, difference in tint is great between the neighbouring area 2b and the remote area 2d while the intermediate area 2c gives a transitive tint. Going into details, the remote area 2d provides emission which looks to be insufficiently bluish as compared with emission from the neighbouring area 2b. The greater depth of the scattering guide plate 2 viewed from the light supply side is, the more remarkable such a tendency is.
Such insufficiency of bluishness has been recognized already to some extent. According to an prior art, blue component of light supplied from the primary light source 3 is reinforced. But this provides no essential solution to the problem of unevenness in tint and may give excessive bluishness to the neighbouring area 2b.
According to another prior art, a blue-coloring agent is additionally contained in the scattering guide plate 2. This manner provides no essential solution either. Graded density may be employed in adding of the blue-coloring agent. However, it is disadvantageous to production technology and costs much.
Such a problem of unevenness in tint can rises in light source devices which employ scattering guides having other shapes. FIG. 2 illustrates a light source device which employs a rod-shaped scattering guide (scattering guide rod). The illustrated light source device 11 comprises a cylindrical scattering guide rod 12 having an end face 12a beside which a primary light source (e.g. fluorescent lamp) 13 is disposed. And the primary light source 13 supplies light to the scattering guide rod 12. Emission is outputted from a side face of the cylinder and a distal end face 12e.
This light source device 11 also shows unevenness in tint, which has tendency similar to that shown in the case of the light source device 1 illustrated in FIG. 1.
For the sake of explanation, the side face of the scattering guide 12 providing a cylindrical emission face is divided into three areas, neighbouring area 12b near to the end face portion 12a, remote area 12d far from the end face portion 12a and intermediate area 12c located between them.
Difference in tint is great between the neighbouring area 12b and the remote area 12d (and the distal end face 12e). In the intermediate area 12c gives a transitive tint. Going into details, the remote area 12d (and distal end 12e) provides emission which looks to be insufficiently bluish as compared with emission from the neighbouring area 12b. The greater length (i.e. depth viewed from the light supply side) of the scattering guide rod 12 is, the more remarkable such a tendency is.